1. Field of the Invention
The present invention is in the field of semiconductor manufacturing and specifically relates to apparatus for sensing when a wafer is present and properly seated on the carrier of a wafer polishing machine, which is a sophisticated numerically-controlled machine that greatly reduces the need for intervention by a human operator. A typical wafer is several inches in diameter and about 680 microns thick; one micron is 1 millionth of meter.
In the polishing machine, the movement of the wafer carrier is programmed to acquire a wafer from a first station, to transport the wafer to a polishing surface, to drive the wafer across the rotating polishing surface, to carry the wafer from the polishing surface to the second station, and to release the wafer at the second station. The present invention provides a way of determining, prior to each of these automated operations, whether the wafer is, in fact, present and in its proper place on the carrier.
2. The Prior Art
Pertinent aspects of the polishing machine are described in U.S. Pat. No. 5,423,716 issued Jun. 13, 1995 to Strasbaugh. That patent is incorporated herein by reference to avoid unnecessary repetition; the patent provides valuable background for a better understanding of the present invention.
Briefly, U.S. Pat. No. 5,423,716 describes the use of a resilient diaphragm to acquire and hold a wafer that is being processed. The carrier includes a downwardly opening plenum, and the resilient diaphragm covers the opening. The wafer to be acquired is elevated into contact with the resilient diaphragm. A vacuum is then applied to the plenum, causing the diaphragm to bulge into the plenum and to apprehend the upper surface of the wafer by means of a "suction cup" effect. So long as the vacuum is maintained, the wafer remains held against the underside of the carrier. This condition is maintained while the wafer is being carried by the carrier to the polishing pad. When it is desired to remove a uniform thickness of material across the face of a wafer, the vacuum is relieved and an overpressure is applied to the plenum. After having been polished, the wafer must be transported from the polishing pad to another station, and to accomplish this a vacuum is again applied to the plenum.
Although the "suction cup" technique described in U.S. Pat. No. 5,423,716 can be applied in an autonomous machine with success, subsequent experience has shown that occasionally the wafer is not acquired or does not seat properly on the carrier. In some instances the desired seal between the resilient diaphragm and the wafer does not form, or gives way, causing the carrier to leave the wafer behind or to drop the wafer in transit. When any of these rare circumstances occurs, it is desirable to stop the operation of the machine and to produce an alarm so that a human operator can intervene.